(1) Field of the Invention
This invention relates to radiographic apparatus used in medical, industrial and other fields for producing sectional images of patients or objects under examination. More particularly, the invention relates to a technique for reducing artifacts appearing in the sectional images.
(2) Description of the Related Art
Conventional radiographic apparatus include a CT (Computed Tomography) type X-ray radiographic apparatus (hereinafter called X-ray CT apparatus where appropriate), for example, which has made remarkable progress in recent years. This X-ray CT apparatus has an X-ray tube and an image intensifier opposed to each other across an object under examination. The X-ray tube emits X rays in the form of a cone beam to the object, and the image intensifier two-dimensionally detects X rays transmitted through the object. Radiography is performed while synchronously driving the X-ray tube and image intensifier to make one revolution (at least a half revolution) in a single plane about a scan axis set substantially centrally of a region of interest of the object and extending perpendicular to that plane. This operation acquires transmitted images for one revolution (at least a half revolution) about the body axis of the object.
This X-ray CT apparatus carries out an image reconstruction, by using the Feldkamp method, from the plurality of transmitted images acquired, to produce three-dimensional volume data of the region of interest of the object. When a sectional plane is selected from this three-dimensional volume data, the selected sectional image (sectional image seen in a direction along the body axis of the object) is displayed on a monitor or the like.
In this way, three-dimensional volume data of the region of interest is obtained in one radiographic operation. This provides an advantage that an image of a desired sectional plane may be displayed quickly after the radiographic operation, simply by selecting the sectional plane.
However, the conventional apparatus noted above has the following drawback. With the image reconstruction using the Feldkamp method, artifacts appear in a reconstructed image of a position in the more pronounced way, the farther away along the scan axis that position is from a center plane located substantially centrally of the region of interest of the object and extending perpendicular to the scan axis. This is believed due to a volume scan mode as described below.
As noted above, the X-ray tube emits X rays in the form of a cone beam to the object, and the image intensifier two-dimensionally detects X rays transmitted through the object. An image reconstruction is carried out, by using the Feldkamp method, based on transmitted images detected in varied scan positions. For example, point-to-point paths extending between the center of cone beam X rays emitted from the X-ray tube and a pixel located on the detecting plane of the image intensifier substantially centrally along the scan axis are in the same slice plane, that is the point-to-point paths are in agreement, for different scan positions. Thus, a reconstructed sectional image of a substantially center plane of a three-dimensional volume is free from artifacts due to a disagreement of point-to-point paths. However, point-to-point paths extending between the center of cone beam X rays emitted from the X-ray tube and a pixel located on the detecting plane of the image intensifier away from the center along the scan axis, i.e. in a direction of divergence (direction of inclination) of the cone beam X rays, are not in the same slice plane, but are different for different scan positions. Thus, artifacts due to a disagreement among the point-to-point paths appear in a reconstructed image of a position in the more pronounced way, the farther away that position is from the center plane of the three-dimensional volume.
This invention has been made having regard to the state of the art noted above, and its object is to provide a radio-graphic apparatus that suppresses artifacts and the like due to the volume scan mode.
The above object is fulfilled, according to this invention, by a radiographic apparatus for obtaining sectional images from three-dimensional volume data of a region of interest of an object under examination generated by an image reconstruction of projection data acquired by radiographing the object from varied scan positions, the apparatus comprising:
a radiation source for irradiating the object with penetrating electromagnetic waves in form of a divergent beam;
an area detector opposed to the radiation source across the object for detecting electromagnetic waves transmitted through the object;
a scanning device for causing the radiation source and the area detector synchronously to revolve in an identical plane about a scan axis set substantially centrally of the region of interest;
an image processor for performing a predetermined image processing on projection data detected in the varied scan positions; and
a back projection unit for performing the image reconstruction to generate three-dimensional volume data of the region of interest by projecting the projection data processed by the image processor back to predetermined lattice points of a three-dimensional lattice virtually set to the region of interest of the object radiographed;
the image processor applying a low-pass filtering to projection data in each row of pixels of the area detector perpendicular to a direction corresponding to the scan axis, the low-pass filtering being in accordance with a location on the scan axis to which each row of pixels is projected.
With this apparatus, the scanning device causes the radiation source for irradiating the object with penetrating electromagnetic waves in form of a divergent beam and the area detector opposed to the radiation source across the object for detecting electromagnetic waves transmitted through the object, synchronously to revolve in an identical plane about a scan axis set substantially centrally of the region of interest. The image processor applies a low-pass filtering to projection data in each row of pixels of the area detector perpendicular to a direction corresponding to the scan axis, the low-pass filtering being in accordance with a location on the scan axis to which each row of pixels is projected. The back projection unit performs an image reconstruction to generate three-dimensional volume data of the region of interest by projecting the projection data processed by the image processor back to predetermined lattice points of a three-dimensional dimensional lattice virtually set to the region of interest of the object radiographed. In this way, an appropriate low-pass filter is applied to reduce artifacts that would appear in a position in the more pronounced way, the farther away along the scan axis that position is from a center plane located substantially centrally of the region of interest of the object and extending perpendicular to the scan axis. Thus, artifacts are suppressed from appearing in positions of a reconstructed image remote along the scan axis.
Preferably, the image processor is arranged to apply a low-pass filtering to pass the lower frequency for projection data in a row of pixels of the area detector the farther away from an emission reference line extending perpendicular to the scan axis from a beam center of the radiation source. In this way, an appropriate low-pass filter is applied to reduce artifacts that would appear in a position in the more pronounced way, the farther away along the scan axis that position is from a center plane located substantially centrally of the region of interest of the object and extending perpendicular to the scan axis. Thus, artifacts are suppressed from appearing in positions of a reconstructed image remote along the scan axis.
Preferably, the image processor is arranged to apply a low-pass filtering as a diffusion proportional to sin(xcex1), where xcex1 is an angle of divergence formed between an emission reference line extending perpendicular to the scan axis from a beam center of the radiation source and a projection line extending from the radiation source and each row of pixels of the area detector. In this way, an appropriate low-pass filter is applied to reduce artifacts that would appear in a position in the more pronounced way, the farther away along the scan axis that position is from a center plane located substantially centrally of the region of interest of the object and extending perpendicular to the scan axis. Thus, artifacts are suppressed from appearing in positions of a reconstructed image remote along the scan axis.
Preferably, the area detector is a flat panel detector with gate lines arranged in a direction corresponding to a direction of the scan axis, the image processor applying the low-pass filtering by simultaneously turning on gates of a predetermined number of rows of pixels corresponding to the direction of the scan axis. In this way, an appropriate low-pass filter is applied to reduce artifacts that would appear in a position in the more pronounced way, the farther away along the scan axis that position is from a center plane located substantially centrally of the region of interest of the object and extending perpendicular to the scan axis. Thus, artifacts are suppressed from appearing in positions of a reconstructed image remote along the scan axis.
Preferably, the scanning device performs, instead of the scanning by revolution in the identical plane, a linear scanning for linearly moving one of the radiation source and the area detector in a first direction perpendicular to the scan axis, and the other synchronously therewith in a second direction parallel and counter to the first direction. Thus, also in a non-CT type radiography (in which the radiation source and area detector are not caused to make more than a half revolution about the body axis of the object) which causes the radiation source and area detector to move linearly, parallel to each other, and scan the object lying in between, and carries out an image reconstruction to generate three-dimensional volume data of a region of interest of the object, artifacts are suppressed from being reconstructed in positions having varied angles of divergence.
Preferably, two arcuate tracks are set on a circumferential track around the object to be opposed to each other across the object such that a straight line between midpoints of the two arcuate tracks coincides with the scan axis, the scanning device performing, instead of the scanning by revolution in the identical plane, an arcuate scanning for moving the radiation source on one of the arcuate tracks, and the area detector on the other arcuate track synchronously therewith to maintain a fixed distance from the radiation source. Thus, also in a non-CT type radiography which causes the radiation source and area detector to move separately and arcuately and scan the object lying in between, and carries out an image reconstruction to generate three-dimensional volume data of a region of interest of the object, artifacts are suppressed from being reconstructed in positions having varied angles of divergence.
Preferably, the scanning device performs, instead of the scanning by revolution in the identical plane, a circular scanning for revolving the radiation source in one of parallel planes opposed to each other across the object and extending perpendicular to the scan axis, and the area detector in the other plane synchronously therewith and in a direction opposite to a direction of revolution of the radiation source. Thus, also in a non-CT type radiography which causes the radiation source and area detector to revolve separately in the two parallel planes and scan the object lying in between, and carries out an image reconstruction to generate three-dimensional volume data of a region of interest of the object, artifacts are suppressed from being reconstructed in positions having varied angles of divergence.